The present invention relates generally to the operation of an evaporator in a heating and cooling system or process system, and more specifically, to the operation of a falling film evaporator in a two-phase refrigerant heating and cooling system or process system.
Certain process systems, as well as heating and cooling systems for buildings or other structures that typically maintain temperature control in a structure, circulate a fluid within coiled tubes such that passing another fluid over the tubes effects a transfer of thermal energy between the two fluids. A primary component in such a heating and cooling system is an evaporator that includes a shell with a plurality of tubes forming a tube bundle through which a secondary fluid, such as water or ethylene glycol, is circulated. A primary fluid or refrigerant, such as R134a, is brought into contact with the outer or exterior surfaces of the tube bundle inside the evaporator shell resulting in a thermal energy transfer between the secondary fluid and the refrigerant. In a typical two-phase heating and cooling system, the refrigerant is heated and converted to a vapor state, which is then returned to a compressor where the vapor is compressed, to begin another refrigerant cycle. The secondary fluid, which has been cooled, is circulated to a plurality of coils located throughout the building. Warmer air is passed over the coils where the secondary fluid is being warmed while cooling the air for the building, and then returns to the evaporator be cooled again and to repeat the process.
Evaporators with refrigerant boiling outside the tubes include flooded evaporators, falling film evaporators and hybrid falling film evaporators. In conventional flooded evaporators, the shell is partially filled with a pool of boiling liquid refrigerant in which the tube bundle is immersed. Therefore, a considerable amount of the refrigerant fluid is required, which is costly to provide, and may be an environmental and/or safety concern, depending upon the composition of the refrigerant, in case of leakage of the refrigerant from the evaporator or from the whole system, in which the whole charge of refrigerant may be lost. Therefore, it is desired to reduce the charge of refrigerant in the system.
In a falling film evaporator, a dispenser deposits, such as by spraying, an amount of liquid refrigerant onto the surfaces of the tubes of the tube bundle from a position above the tube bundle, forming a layer (or film) of liquid refrigerant on the tube surface. The refrigerant in a liquid or two-phase liquid and vapor state contacts the upper tube surfaces of the tube bundle, and by force of gravity, falls vertically onto the tube surfaces of lower disposed tubes. Since the dispensed fluid layer is the source of the fluid that is in contact with the tube surfaces of the tube bundle, the amount of fluid required inside the shell is significantly reduced. However, there are technical challenges associated with the efficient operation of the falling film evaporator.
One challenge is that a portion of the fluid vaporizes and significantly expands in volume. The vaporized fluid expands in all directions, causing cross flow, or travel by the vaporized fluid in a direction that is transverse, or at least partially transverse to the vertical flow direction of the liquid fluid under the effect of gravity. Due to the cross flow disrupting the vertical flow of the fluid, at least a portion of the tubes, especially the lower positioned tubes of the tube bundle, receive insufficient wetting, providing significantly reduced heat transfer with the secondary fluid flowing inside those tubes in the tube bundle.
One attempted solution to this problem associated with falling film evaporators is U.S. Pat. No. 6,293,112 (the '112 patent). The '112 patent is directed to a falling film evaporator wherein the tubes of the tube bundle are arranged to form vapor lanes. The purpose of the vapor lanes is to provide access paths for the expanding vaporizing fluid so that the vertically downward flow of liquid refrigerant is not substantially impacted. In other words, the access paths are provided to reduce the effect of cross flow caused by expanding vaporizing fluid. Thus, the '112 patent has identified that cross flow caused by expanding vaporizing fluid necessarily occurs.
Another challenge is the compressor, which receives its supply of vaporized fluid from an outlet typically formed in the upper portion of the evaporator, can be damaged if the vaporized fluid contains entrained liquid droplets. Since the vaporized fluid adjacent the upper portion of the tube bundle typically contains these entrained liquid droplets, which would otherwise be drawn into the compressor, components must be implemented to provide separation between the vapor and liquid droplets. These components include, for example, a means to provide impingement of the liquid droplets, such as a baffle or mesh, a volume within the evaporator, which typically requires about one half of the volume of the evaporator, for gravity separation of the liquid droplets, or the impingement means in combination with the gravity separating volume. However, each of these components and combinations thereof add to the complexity and cost of the system, and may also result in an undesired pressure drop prior to the vapor refrigerant reaching the compressor.
A further challenge associated with falling film evaporators concerns the distributor, which is located in an upper portion of the evaporator shell. Refrigerant applied by the distributor at high pressure and/or two-phase liquid and vapor tends to generate mist and fine liquid droplets, in addition to those generated by the evaporation of the liquid on the tube bundle. Being generated in the upper portion of the evaporator shell, these droplets are easily entrained into compressor suction. Thus, many designs require a combination of a device to lower the pressure of the fluid before the distributors, and of a device to separate the vapor from the liquid before the distributor in order to very gently deposit liquid on top of the tube bundle.
A brochure produced by Witt GmbH, entitled “Instruction Guide for the BVKF type, updated November, 1998” is directed to a falling film evaporator that has a sheet metal hood with diverging walls positioned over the tube bundle and refrigerant distribution nozzles. The hood covers the tube bundle and extends partially along the sides of the bundle and directs refrigerant vapor with entrained droplets around the hood such that the droplets will have additional opportunity to separate from the gas flow as gas rises outside the hood toward the evaporator discharge. However, this concept does not prevent cross flow caused by expanding vaporizing fluid.
Finally, a hybrid falling film evaporator incorporates the attributes of a falling film evaporator and a flooded evaporator by immersing a lesser proportion of the tubes of the tube bundle than the flooded evaporator while still spraying fluid on the upper tubes, similar to a falling film evaporator.
What is needed is a falling film evaporator that substantially prevents cross flow caused by expanding vaporizing fluid and which also requires less space than a flooded evaporator for liquid droplet separation than a conventional flooded or existing designs of flooded film or hybrid evaporators.